1. Field of the Invention
The present invention relates to a method for simultaneous amplitude and quantitative phase contrast imaging by numerical reconstruction of digitally encoded holograms.
2. Discussion of the Background
The idea of recording a hologram of the specimen and reconstructing the hologram with a numerical method has been reported for the first time in 1967 by J. W. Goodman and R. W. Laurence, "Digital image formation from electronically detected holograms," Appl. Phys. Lett. 11, 77-79 (1967). who used a vidicon detector for the hologram recording, and in 1971 by M. A. Kronrod et al., "Reconstruction of a hologram with a computer," Soviet Phys.--Technical Phys. 17, 333-334 (1972) who used a digitized image of a hologram recorded on a photographic plate. In these two references, the holograms were recorded in the holographic Fourier configuration for which amplitude contrast images of the specimen can be numerically reconstructed by simply calculating the modulus of the two-dimensional Fourier transform of the hologram. A recent development using a Charged Coupled Device (CCD) camera as recording device, also in the Fourier configuration, has been patented (U.S. Pat. No. 4,955,974 and U.S. Pat. No. 5,214,581) and reported by W. S. Haddad et al., "Fourier-transform holographic microscope," Applied Optics 31, 4973-4978 (1992)., and by K. Boyer et al., "Biomedical three-dimensional holographic microimaging at visible, ultraviolet and X-ray wavelength," Nature Medicine 2, 939-941 (1996)., for X-ray amplitude contrast imaging of biological specimens.
In 1994, on the basis of precedent works about digital holography, L. P. Yaroslavskii and N. S. Merzlyakov, Methods of Digital Holography, (Consultants Bureau. New York, 1980)., Schnars et al., "Direct recording of holograms by a CCD target and numerical reconstruction," Applied Optics 33, 179-181 (1994). have reported the first numerically reconstructed amplitude contrast image from an off-axis hologram recorded in the Fresnel holographic configuration with a CCD camera. With the same reconstruction algorithm but with a low-coherence light source, E. Cuche et al., "Optical Tomography at the Microscopic scale by means of a Numerical Low Coherence Holographic Technique," Proceedings SPIE on Optical and Imaging techniques for Bioimaging, Vienna, Vol. 2927, 61-66 (1996) have reported an application of numerical holography for tomographic imaging. An application in micro-endoscopy, using also a Fresnel calculation for the hologram reconstruction, has been reported by O. Coquoz et al., "Performances of endoscopic holography with a multicore optical fiber," Applied Optics 34, 7186-7193 (1995).
All the above mentioned works concern amplitude contrast imaging for which only the modulus of the numerically reconstructed optical field is considered. The first use of a numerically reconstructed phase distribution, from a Fresnel hologram, has been reported by Schnars et al., "Direct phase determination in hologram interferometry with use of digitally recorded holograms," J.Opt.Soc.Am.A 11, 2011-2015 (1994). for an application in holographic interferometry. As presented in Ref. Schnars et al., the used reconstruction algorithm is the same as for amplitude contrast imaging and do not really allows phase contrast imaging, because, in this case, the reconstructed optical field is the product of the object wave by the complex conjugate of the reference wave (or the product of the complex conjugate of the object wave by the reference wave). However, this reconstruction algorithm can be used in holographic interferometry, because the subtraction between two reconstructed phase distributions obtained with the same reference wave provides an image which represents only the phase difference between the deformed and undeformed states of the object.
The first example of a numerically reconstructed phase contrast image has been reported in 1997 by E. Cuche et al., "Tomographic optique par une technique d'holographie numerique en faible coherence," J. Opt. 28, 260-264 (1997). who have used a modified reconstruction algorithm including a multiplication of the digital hologram by a digital replica of the reference wave (digital reference wave). In Ref. Cuche et al., the phase contrast image has been obtained with a plane wave as reference and in direct observation, meaning that no magnification or demagnification optics is introduced along the optical path of the object wave.